A current transducer, also known as current sensor, is a device that transforms an electrical current to be measured, also called primary current, into another easily measurable electrical quantity, such as a current of the order of milliamps, or voltage of the order of volts. The current transducers that are subject of this invention belong to the group of isolated current transducers, also known as isolated DC/AC current transducers. Such current transducers
provide electrically isolated current measurement, i.e. the conductor with the primary current is isolated from the output circuit of the transducer;
are capable of measuring both direct currents (DC) and alternating currents (AC);
may have either a fixed geometry, or
can be opened and re-assembled around a cable conducting the primary current, without having to disconnect the cable; such current transducers are known as clip-on, clamp-on, or split-core current transducers.
The principle of operation of the known isolated current transducers is based on the measurement of the magnetic field associated with the current in the enclosed cable. The state-of-the-art of current transducers is described in the paper by Pavel Ripka, “Electric current sensors: a review”, MEASUREMENT SCIENCE AND TECHNOLOGY 21 (2010) 112001, pp. 1-23.
Briefly, an isolated current transducer usually consists of a combination of a ferromagnetic core, magnetic field sensor, and electronic circuit for signal conditioning. The ferromagnetic core, which encloses a current-carrying cable, serves to guide the magnetic flux generated by the electrical current in the cable, and to concentrate this flux on the magnetic field sensor. The magnetic field sensor converts the magnetic field into an electrical signal. The electronic circuit supplies the magnetic field sensor with a suitable electrical current and amplifies, filters, and conditions the signal coming from the magnetic field sensor. In the version of current transducers known as open-loop current transducers, the output signal of the current transducer is the conditioned signal of the magnetic field sensor. There is also a class of current transducers known as closed-loop current transducers. A closed-loop current transducer has a coil wound around the ferromagnetic core, and the electronic circuit is configured to supply a current, called secondary current, to this coil, which compensates the magnetic flux generated by the primary current. The secondary current is the output signal of the closed-loop current transducer.
Measurement accuracy of the known isolated current transducers is severely limited by the sensitivity of the measurement system to external magnetic fields (for example, the magnetic fields associated with the currents in neighbouring cables).
In the following throughout the specification the current to be measured is called primary current. The magnetic field generated by the primary current is called primary magnetic field. Furthermore, the ferromagnetic core is sometimes simply referred to as core.